Various bright tin-nickel alloy plating electrolytes have been proposed in the past which comprise a stannous salt and nickel salt and various other additives which are used for solubilizing and/or complexing the metal salts. In addition, various brightener have been added to the plating liquid such as an amine moiety containing compounds as well as other. Illustrative bath components are disclosed in patents such as U.S. Pat. No. 3,753,873 and U.S. Pat. No. 3,887,444. The last patent discloses nickel-tin alloys plated from alkaline medium containing cyanides which pose a great number of problems with respect to toxicity and effluent treatment. The above-mentioned patents disclose alkaline bath because, as it is well known, cyanide baths cannot be used with an acidic medium.
A tin-cobalt alloy plating from a pyrophosphate bath has been disclosed in Bull. India Sect., Electrochem. Soc., Vol. 9, pp. 13 to 14 (1960). However, the bath does not provide consistent deposits and the preferrential plating of one of the other metals renders the bath undesirable. Moreover, powdery deposits often occur when using pyrophosphate baths of prior art. As a possible composition, a ternary alloy containing tin and nickel with a balance being cobalt is disclosed in the above literature reference.
In general, the prior art bath used for depositing tin-nickel alloys have used fluoride based electrolytes which again pose many problems associated with waste or effluent treatment. Illustrative patents in this group are U.S. Pat. Nos. 3,141,836 and 3,682,604.
While a number of other, non-electrolytic methods are known for depositing tin, such as by electroless or replacement coating for tin, illustrated in U.S. Pat. No. 3,303,029 for nickel such as shown in U.S. Pat. No. 3,265,511, in general, the prior art methods associated with electroless or immersion plating suffer from a number of disadvantages especially because of the failure to obtain proper codeposits of alloys.
While some methods have lately been disclosed in the electroless deposition field, the necessity to improve the electroplating of tin-nickel alloys has become manifestly evident because of the advantages that a good electroplating method provides.
In plating a suitable substrate material to protect it from corrosion, the deposits should be thick enough to be virtually nonporous and thus a good throwing or leveling power of a tin-nickel plate is fairly important. The previously mentioned fluoride bath provides highly stressed hard deposits (750 Knoop units 25 grams) and thus deposits of over 75 microinches are not possible.
While the leveling or throwing power has often been achieved by additives including a great number of amine base additives, the presently discovered combination of elements achieves the same result with fewer elements in a different combination. Moreover, the deposit according to the present invention is ductile, i.e., will not exfoliate in thickness up to 1 mil or over and is softer (400- 450 Knoop value- 25 gr.) as well as is without the undesirable pink cast. Inasmuch as the tin-nickel intermetallics are very useful in the electronic industry as substrate for gold and the corrosion resistance for contacts which are overplated with gold, the present deposits need only e. g. 5 .mu. inches gold whereas prior art nickel underplate requires 30 .mu. inches to achieve equivalent corrosion resistance. The present electrolyte makes it manifestly evident the advantages which can be gathered from a properly coacting element in an electrolytic bath.
Still further, in stripline plating, that is, where a continuous plating on a continuously moving strip is being deposited in a limited area of the substrate such as copper or bronze, nickel-iron; copper-nickel-tin; ferrous or nonferrous materials or other suitable metals, the plating speeds have been achieved which are not normally achieved by rack or barrel plating methods. These speeds are as a result of the high transfer of the plating solution to the area being plated and the removal of the depleted plating solution such as by belts or discs or spiral contact surfaces moving at a rate different from a moving stripline. High current densities are practical such as of densities up to 300 ASF.
Prior art fluoride baths appear to require a current density range of 20 to 30 ASF and any deviation from it results in an alloy deposit of varying proportions of pure tin and nickel. The deposits obtained when using the present bath solution in stripline plating are highly corrosion resistant and are of good quality.
As another advantage over the prior art baths, such as the fluoride solution containing baths, is the considerably simplified waste treatment of the present bath solution without the increasingly stricter fluoride waste treatment requirements which are now required and which require rather elaborate processing steps. For example, in some states the amount of fluoride in the effluent cannot exceed 1 part per million.
Hence, the present bath composition has provided an alternate bath which possesses all of the listed advantages of the prior art baths and also provides simplified waste treatment. The presently discovered bath compositions and the effluents therefrom are readily treated in a simplified manner for meeting the pollution requirements and the effluent dischange is substantially nontoxic.